1. Field of the Invention
The invention relates to the general field of process control as applied to fractional distillation columns used in the chemical, petrochemical and petroleum industries. The invention more specifically relates to a control system and apparatus for controlling a xe2x80x9cdividing wallxe2x80x9d fractional distillation column used to separate a feed stream into three or more product streams.
2. Related Art
Fractionation columns are widely employed to do myriad separations in a number of industries. Control systems for fractionation columns have therefore reached a high state of development including the use of on-line analytical instruments and computerized optimization. They still however, rely to a great extent on the measurement of temperatures and liquid levels in the fractionation column by equipment such as shown in U.S. Pat. No. 3,855,074 issued to H. A. Mosler et. al.
The dividing wall or Petyluk configuration for fractionation columns was initially introduced some 50 years ago by Petyluk et al. Dividing wall columns have been employed for the separation of hydrocarbon mixtures as evidenced by the disclosure of U.S. Pat. No. 2,471,134 issued to R. O. Wright. Recently the use of dividing wall columns has begun to expand because of the greater recognition that in certain situations dividing wall columns can provide benefits above those of conventional fractionation columns. For instance, a commercialization of a fractionation column employing this technique is described in the article appearing at page s14 of a supplement to The Chemical Engineer, Aug. 27, 1992.
The control systems for dividing wall columns are not as mature or as commonly described as for conventional columns. Studies of control variables and responses in a small scale pilot plant column are given in Operation and Control of Dividing Wall Distillation Columns, by A. Mutalib and R. Smith, Part 1: Degrees of Freedom and Dynamic Simulation, Trans. IChemE. Vol. 76, Part A, March 1998, pages 308-318 and Part 2: Simulation and Pilot Plant Studies Using Temperature Control, Trans. IChemE, Vol 76, Part A, March 1998, pages 319-334. Two other papers directed to the general control of dividing wall columns are The Control of Dividing Wall Column (Centre for Process Integration, UMIST, Manchester, UK) by F. Lestak and R. Smith appearing at page 307 of Chemical Engineering Research and Design, Institution of Chemical Engineers (1993) 71 (A3) and Optimizing Control of Petlyuk Distillation: Understanding the Steady-State Behavior by Ivar J. Halvorsen and Sigurd Skogestad at pages s249-s254 of Computers Chem. Engng., Vol. 21, Suppl., 1997 (Elsevier Science Ltd.)
An actual example of a control system for these columns is provided in U.S. Pat. No. 4,230,533 issued to V. A. Giroux. This latter reference is relevant as liquid is collected in an upper portion of the column and divided between the two sides of the dividing wall by level control. The flow rate of vapor upward in the two sides of the column is set by two valves in the lower portion of the column in response to an analyzer measuring the composition on one side of the dividing wall.
The invention is an apparatus and method for controlling the relative flow rate of vapor in the two parallel contacting sections present in the central portion of a dividing wall column. The liquid level on the top tray of each section of the column is varied to control the rate of vapor flow through the respective section of the column. The liquid level on each of these two trays is in turn controlled by differential pressure measurement devices which measure the pressure above and below this tray and determine a pressure differential. Additional liquid flow into the top of the receiving section of the column is set by a temperature controller near this top tray, and the liquid flow into the top of the discharging section of the column is set by a level controller near the top tray.
An apparatus for controlling the relative rate of upward vapor flow in each of two parallel, partial-column contacting sections of a divided wall column, the column being divided into at least said two parallel divided wall contacting sections located in a middle portion of the column by a vertical dividing wall, said apparatus comprising a first differential pressure measuring device measuring the pressure differential between vapor-filled volumes located immediately above and below a first dedicated perforated tray located in an upper portion of a first partial-column contacting section of said column; a second differential pressure measuring device measuring the pressure differential between vapor-filled volumes located immediately above and below a second dedicated perforated tray located in an upper portion of a second partial-column contacting section of said column; a liquid collection tray located in the column above the first and second partial-column contacting sections of the column, the liquid collection tray having a liquid collection well and also having openings for the upward flow of vapor through the tray; a liquid withdrawal and distribution conduit system communicating with the liquid collection well of the liquid collection tray and comprising conduits for the passage of liquid from the liquid collection well to other points in the fractionation column including a first conduit having an outlet discharging onto the first dedicated tray and a second conduit having an outlet discharging onto the second dedicated tray; a first flow control valve located in the first conduit and controlling the rate of fluid flow through the first conduit in response to a signal generated by the first pressure differential measuring device; and, a second flow control valve located in the second conduit and controlling the rate of fluid flow through the second conduit in response to a signal generated by the second pressure differential measuring device.